Ion-getter vacuum pump and gauge



INVENTOR CHARLES L. HALL ATTORNEY June 27, 1967 c. L. HALL ION'GBTTER VACUUM PUMP AND GAUGE Filed Sept. 15, 1965 s ll' m I VlfIl'r/IIIIII Ill!!! rill/ United States Patent 3,327,931 KUN GETTER VACUUM PUMP AND GAUGE Charles L. Hall, 925 Ruble Ridge, Palo Alto, Calif. 94306 Filed Sept. 13, 1965, Ser. No. 486,649 14- Claims. (Cl. 230-69) ABSTRAQT 0F THE DISCLOSURE The ion-getter vacuum pump of the present invention utilizes a cylindrically shaped ion collector for surrounding one or more centrally disposed hairpin-shaped tungsten alloy filaments which are heated to emit electrons. The ion collector is maintained at a potential which is negative with respect to the potential of the filaments so that the emitted electrons are focussed along the axis of the ion collector to form a collimated electron stream. One or more pellets of getter material are positioned, in close proximity to the end face of the ion collector, in the electron stream for bombardment to sublime the getter material when connected to become the anode.

When more than one getter material pellet is used, only one such pellet is connected to the positive high voltage supply at any one time to become the anode. After a pellet is consumed after many hours of use, the next pellet is connected to the high voltage supply to become the anode and to now furnish the sublimate. In this manner, each individual pellet may be selected of sufliciently small size for efficient sublimation, and by providing additional pellets with suitable lead-through for connections to the high voltage power supply, the lifetime of the ion-getter vacuum pump may be made as long as desired.

This invention relates to high vacuum pumping and gauging devices and more particularly to an ion-getter vacuum pump in which electrons, emitted from a thermionic cathode, produce very high vacuums by ionizing residual gas molecules which are impelled by electric fields and/or by collisions to move against the walls of the pump and/or ion collector where they are permanently fixed by burying under a coating of a sublimed getter material.

There have been a number of prior art devicesutilizing the deposited sublimate of a getter material for fixing and permanently burying captured ions, atoms or molecules for pumping action. One such prior art ion pump, described in United States Letters Patent 2,727,167, issued on Dec. 13, 1955, to D. Alpert, utilizes an axial filament for continually emitting electrons to ionize the residual gas, and another filament, made of or coated with one of the well-known getter materials, to sublime the getter material whenever the pumping action decreases.

Another prior art device, instead of making use of a heated getter material filament, utilizes a large wire of getter material which is fed, by a complex feeding mechanism, into a crucible where it is bombarded by the ionizing electrons, melted and evaporated, to form the vapor for condensation upon the interior walls. Such a pump is described in United States Letters Patent 2,894, 679, issued on July 14, 1959, to R. G. Herb.

While both of the above described prior art getter-ion pumps provide satisfactory pumping action by ionizing, capturing and burying ions, each has certain disadvantages which the present invention seeks to overcome. For example, the filamentary getter material wire disclosed by Alpert requires additional lead-through connections and an additional source of power which increases the cost of the ion pump. Further, since the getter material filament 332733 1 Patented June 27, 1967 must be very thin to develop the necessary sublimation temperature upon being connected to a conventional filament power supply and is being consumed, the lifetime of such a pump is severely limited. Additionally, as the filament is being consumed, the filamentary current must be increased to compensate for the increased filament resistance. The ion pump disclosed by Herb overcomes these problems but introduces others equally severe. The presence of molten titanium causes rapid deterioration of all structural elements with which it comes in contact. Furthermore, the feeding mechanism, because of its complexity, adds materially to the expense of the pump and markedly decreases the reliability of the pump. Because of the limited life and reliability of the Herb device, it has never become widely accepted.

It is, therefore, a primary object of this invention to provide an improved ion-getter vacuum pump.

It is a further object of this invention to provide an ion-getter vacuum pump which is simple in construction, reliable in operation, has a long life, and is inexpensive to produce.

It is a further object of this invention to provide an ion-getter vacuum pump which is highly efficient and which utilizes the ion collector for focussing the ionizing electrons for bombarding the getter material to form the sublimate, thereby decreasing the power requirements norm-ally necessary to operate ion-getter vacuum pumps.

It is a further object of this invention to provide an ion-getter vacuum pump and gauge which has a lifetime equal to the combined lifetime of many conventional pumps at the sacrifice of an insignificant increase in manufacturing cost.

Briefly, the ion-getter vacuum pump of this invention utilizes a cylindrically shaped i-on collector for surrounding one or more centrally disposed hairpin-shaped tungsten alloy filaments which are heated to emit electrons. The ion collector is maintained at a potential which is negative with respect to the potential of the filaments so that the emitted electrons are focussed along the axis of the ion collector to form a collimated electron stream. A suitable meter in series with the ion collector allows the pump to be used as a vacuum gauge. The ion current captured by the collector is directly proportional to the pressure.

One or more pellets of getter material are positioned, in close proximity to the end face of the ion collector, in the electron stream for bombardment to sublime the getter material when connected to become the anode.

When more than one getter material pellet is used, only one such pellet is connected to the positive high voltage supply at any one time to become the anode and to furnish the sublimate. After a pellet is consumed after many hours of use, the next pellet is connected to the high voltage supply to become the anode and to now furnish the sublimate. In this manner, each individual pellet may be selected of sufficiently small size for efficient sublimation, and by providing additional pellets with suitable leadthrough for connections to the high voltage power supply, the lifetime of the ion-getter vacuum pump may be made as long as desired.

Further objects and advantages of the present invention will become apparent to those skilled in the art to which the invention pertains as the ensuing description proceeds.

The feasures of novelty that are considered characteristic of this invention are set forth with particularly in the appended claims. The organization and method of operation of the invention itself will best be understood from the following description when read in connection with the accompanying drawings in which:

FIGURE 1 is a somewhat diagrammatical, elevational view, partially in cross section, of a preferred embodiment of the ion-getter vacuum pump of this invention;

3 FIGURE 2 is a view taken along line 22 of FIGURE FIGURE 3 is a view, similar to that of FIGURE 2, but showing an increased number of getter material pellets; and

FIGURE 4 is a cross sectional elevational view of the ion collector showing the getter material pellets in an alternate position.

Referring now to the drawings and more particularly to FIGURES 1 and 2 thereof, there is shown an iongetter vacuum pump 10 including a vacuum tight tube envelope 11 which has a press 12 of the conventional type to seal its lower end portion. Lead-throughs 1318 pass through and are hermetically sealed by press 12. A connecting duct 20 has one end communicated with the interior space 19 of envelope 11 and its other end is connected, in a vacuum tight manner, to the enclosure to be evacuated (or whose vacuum presure is to be measured).

The enclosure to which duct 20 is connected generally includes, or is connected to, a suitable preliminary pumping device capable of reducing the vacuum pressure in the enclosure to about 10- torr or less. The ion-getter vacuum pump of this invention is most effective in the range from 10* torr to below 10- torr and takes over after the vacuum pressure has been lowered to about 10 or 10 torr.

Press 12 of tube envelope 11 supports a support rod 22 which in turn supports an ion collector 23 which is of hollow cylindrical shape and which may either be made of thin nickel sheet or of nickel wire mesh. Electrical connection to ion collector 23 is made by a wire lead 24 which connects the collector to lead-through 14.

Positioned inside ion collector 23 are a pair of hairpin filaments 25 and 26 which are supported by, and electrically connected to, lead-throughs 15, 16 and 17, leadthrough 15 being common to both filaments.

Lead-throughs 13 and 18 in press 12 each have a support rod extension, respectively shown at 27 and 28, which pass through interior space 19 and outside of collector 23. The upper ends of the support rods are bent to extend into the space just above the open end face 29 of collector 23. Pellets 32 and 33 of a reactive getter material, such as titanium, chromium, yttrium or the like, are Welded or otherwise conductively affixed to the ends of support rods 27 and 28.

As will become clearer hereinafter, the positioning of pellets 32 and 33 is important for optimum operation of the ion-getter vacuum pump of this invention. Generally speaking, it is desirable to place each pellet as close to axis of ion collector 23 as possible, but not so close as to cause a spark discharge when one of the pellets is connected to a high voltage supply to form an anode and the other pellet is maintained at a potential which is close to the potential of filaments 25 or 26. For most uniform operation it is also desirable that the spacing of pellets 32 and 33 be symmetrical with respect to the collector axis and with respect to the plane of the collector end face 29, As far as vertical spacing of the pellets from end face 29 is concerned, satisfactory operation is obtained when the pellets are located within a spherical space 30 which is centered at the point of intersection of the collector axis with the plane of end face 29 and which has a radius substantially equal to the internal radius of collector 23. It is to be understood, however, that this range of pellet positioning is for optimum operation, and that the placement of the pellets a small distance outside of the above defined spherical space would likewise provide adequate pumping.

In operation, either filaments 25 or 26 are connected, through lead-throughs 15 and 16, or 16 and 17, to a suitable power supply (not shown) which typically supplies 3 amperes at 2 or 3 volts and which heats up the selected filament to provide copious electron emission. Two filaments are provided so that, in case of burn-out of one, there is a spare filament and the pump need not be replaced. Ion collector 23 is connected, via lead-through 14,

to a source of a suitable reference potential (not shown) which is typically 20 to 50 volts below the filament supply voltage. One of the getter material pellets, 32 or 33, is connected, via lead-through 13 or 18- to a high voltage supply (not shown) which is typically 4,000 to 5,000 volts. In this manner the heated filament becomes the cathode and the connected pellet becomes the anode of this pump.

The electrons emitted from the selected hairpin-shaped tungsten alloy filament are accelerated towards the selected pellet and heat the same by bombardment. The reference potential at which the ion collector is maintained is care fully selected for focussing the emitted electrons into a substantially collimated stream so that a larger number of the emitted electrons are utilized for impacting upon the pellet for more efficient operation. As a result of this bombardment, the pellet serving as anode is sublimed, and the sublimate (vapor) formed is deposited upon the walls of envelope 11 and on the walls of collector 23 to provide ion-getter action.

As is well-known, the sublimate of the getter material traps the ionic or molecular gas residue in the normal fashion. Noble gases such as helium, argon and the like are pumped by ionization, burial and subsequent plastering over with sublimed titanium. The ionization is produced by collision of the filament emitted electrons with the gas molecules.

The focussing achieved in the present invention by 10- eating the filaments inside and along the axis of tubular ion collector 23 and by biasing collecter 23 below the filament voltage has been found to be very efiicient. This geometry provides an axially focussed electron stream which is symmetrical with respect to the axis of collector 23 and has a cross section sufficiently large to permit the placement of a plurality of targets about the collector axis without the necessity of increasing the power input to the filament.

The purpose of utilizing a plurality of getter material pellets is to extend the lifetime of the pump without increasing the power requirement for sublimation. The larger the physical size of a pellet, the longer is the useful life of the pump since it will take a longer time to sublime a large pellet than a small pellet. However, the energy required to subi-me a large pellet is considerably greater than the energy required to sublime a small pellet since the total amount of energy necessary for heating a large pellet is greater than for a small pellet. Accordingly, by utilizing a plurality of small pellets, the total amount of getter materia in the pump may be made equal to that of a large pellet, and by bombarding each small pellet in turn, the energy required to fonm the sublimate is not increased and useful life of the pump is extended many times.

In an ion-getter vacuum pump constructed in accordance with this invention, two pellets, each weighing about milligrams were found to provide a useful pump lifetime of over 10,000 hours in continuous operation at a vacuum pressure of 10* torr with a pumping speed of about 0.2 liter per second and a filament emission rate of 0.5 milliampere.

It will now readily be understood that pellets 32 and 33 should be disposed symmetrically with respect to the focussed electron stream, so that, when the high positive voltage is switched from one to the other pellet, the operation remains substantially unchanged. If the pellets were asymmetrically spaced with respect to the electron stream, then the number and the energy of the bombarding electrons would be different for different pellets resulting in a different sublimation rate.

The placement of the pellets with respect to plane of end face 29 may be varied within certain limits as heretofore explained. The following considerations are pertinent. The closer the pellets are placed to the filaments (and therefore below end face 29 and into the ion collector 23), the smaller is the power requirement to sublime the pellet but the area of getter vapor deposition is also smaller since the collector acts as a shield. Conversely, the more distant the pellets are from the filament (and therefore above end face 29 and out of the ion collector 23), the greater is the power requirement for subliming but the area of vapor deposit is increased. Spherical space 30 has been found to present a good compromise between these conflicting requirements, i.e., small power and large area of vapor deposition.

Device is used as a vacuum gauge by connecting a suitable current meter in series with lead-through 14 to measure the ion current captured by collector 23. The ion current is directly proportional to the vacuum pressure since ion generation is proportional to the number of gas molecules present.

FIGURE 3 shows a cross sectional top view of an ion pump 40 having an envelope 41, and including three symmetrically spaced pellets 42, 43 and 44 of getter material as heretofore described. Instead of utilizing three pellets, it will be readily understood that a larger number may be employed to either decrease the power requirement of the pump by making each pellet smaller, or to increase the lifetime of the pump by providing a larger total quantity of getter material, or both.

FIGURE 4 shows an ion collector sleeve 50 and a pair of support rods 51 and 52 respectively, each of which carries a getter mate-rial pellet indicated at 53 and 54 which are positioned just below the plane of the end face 55 of collector 50. c

There has been described an ion-getter vacuum pump which accomplishes the objects set forth above. The iongetter pump of this invention utilizes an ion collector, biased below the filament potential, to focuse the emitted electrodes into an electron stream for ionizing the residual gas and for subliming a plurality of pellets of getter material, one at a time. In order to reduce the power requirement and extend the lifetime of the pump, each pellet becomes an anode in turn to furnish the sublimate to bury and/ or fix the trapped particles.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A getter-ion vacuum pump comprising:

an enclosure defining an interior space having interior walls and including a port for connecting said interior space to a device to be pumped;

a hollow elongated electrode disposed within said interior space and maintained at a reference potential for collecting ions and for focusing electrons;

an electon emitting filament substantially centrally disposed within said electrode and maintained at a potential which is positive with respect to said reference potential to serve as a cathode, said filament potential being selected for focussing of the emitted electrons by said electrode into anelect-ron stream;

at least two pellets of a getter material, for forming a sublimate for deposit upon said interior walls and the walls of said electrode, disposed within the electron stream; and

means for independently connecting each of said pellets, one at a time, to a potential which is positive with respect to said filament to serve as an anode and to be sublimed by the emitted electrons by bombard ment. 2. A getter-ion vacuum pump in accordance with claim 1 in which said electrode is of cylindrical configuration and in which said pellets are disposed in close proximity to one end face of said electrode.

3. A getter-ion vacuum pump comprising:

an enclosure defining an interior space having interior walls and including a port for connecting said interior space to a device to be pumped;

a hollow elongated electrode disposed within said interior space andmaintained at a reference potential for collecting ions and for focussing electrons, said electrode having an end face lying in a plane substantially at right angles to its axis of elongation and the point of intersection of the axis of elongation with said plane defining a reference point;

an electron emitting filament substantially centrally disposed Within said electrode and maintained at a potential positive with respect to said reference potential to serve as a cathode, said filament potential being selected for optimum focussing of the electrons emitted from said filament;

at least two pellets of a getter material to be sublimed upon said interior walls and the walls of said electrode disposed within a spherical space centered at said reference point and having a radius equal to the radius of said electrode to serve as target for the electrons emitted by said filament and focussed by said electrode; and

means for sequentially connecting each of said pellets, in turn, to a potential which is positive with respect to said filament to serve as an anode for sublimation by the focussed electrons by bombardment until exhausting of said getter material.

4. A getter-ion vacuum pump in accordance with claim 3 in which said pellets are disposed equidistant from the axis of elongation of said electrode.

5. A getter-ion vacuum pump in accordance with claim 3 in which said pellets are disposed in a common plane which is parallel to the plane of said end face.

6. A getter-ion vacuum pump in accordance with claim 3 in which said pellets are disposed in a common plane parallel to the plane of said end face and are equidistant from axis of elongation of said electrode.

7. A getter-ion vacuum pump in accordance with claim 3 in which said electrode is of cylindrical shape and has a non-perforated wall.

8. A getter-ion vacuum pump in accordance with claim 3 in which said electrode is of cylindrical shape and constructed of wire mesh.

9. A getter-ion vacuum pump comprising:

an enclosure defining an interior space and including a port for connecting said interior space to a device to be pumped;

a hollow right cylindrical electrode disposed within said interior space and maintained at a reference potential to serve as an ion collector, the point of intersection of the axis of said electrode with the plane of one end facedefining a reference point;

an electron emitting filament substantially centrally disposed within the interior of said cylindrical electrode and maintained at a potential which is positive with respect to said reference potential to serve as a cathode, the filament potential being selected for optimum focussing of the electrons emitted from said filament; and i at least two pellets of a getter material for forming a sublimate for deposit upon the walls of said interior space and cylindrical electrode, said pellets being disposed within a spherical space centered at said reference point and having a radius equal to the radius of said cylindrical electrode, said pellets being independently connectible to a potential which is positive with respect to said filament to serve, one at a time, as sublimation targets for bombardment by the electrons emitted by said filament and focussed by said cylindrical electrode.

10. A getter-ion vacuum pump comprising:

an enclosure defining an interior space and including a port for connecting said interior space to a device to be pumped;

a hollow right cylindrical electrode disposed within said interior space and maintained at a reference potential to serve as an ion collector, the point of intersection of the axis of said electrode with the plane of one end face defining a reference point;

an electron emitting filament substantially centrally disposed within the interior of said cylindrical electrode and maintained at a potential which is positive with respect to said reference potential to serve as a cathode, the filament potential being selected for optimum focussing of the electrons emitted from said filament;

at least two pellets of a getter material for forming a sublimate for deposit upon the walls of said interior space and cylindrical electrode, said pellets being disposed within a spherical space centered at said reference point and having a radius equal to the radius of said cylindrical electrode, said pellets being independently connectible to a potential which is positive with respect to said filament to serve as sublimation targets for the electrons emitted by said filament and focussed by said cylindrical electrode; and

means for connecting each pellet in turn to the last mentioned potential, said means being switchable upon exhaustion of the getter material to another pellet.

11. A getter-ion vacuum pump comprising:

a housing from which gas is to be removed;

an elongated hollow electrode disposed within said housing and defining an ion collector;

at least one electron emissive filament disposed within the interior of said hollow electrode, said electrode being maintained at a potential which is negative with respect to said filament to focus the electron emitted by said filament along the axis of elongation of said electrode to form an axially focussed electron stream;

at least two pellets of getter material disposed in, and symmetrical with respect to axis of, said electron stream;

means for independently connecting each of said pellets to a source of potential which is positive with respect to said filament to serve as an anode, said stream of electrons sublimating the pellet serving as anode by electron bombardment and ionizing the gas molecules within said housing for driving said ionized gas to the walls of said housing and ion collector where the gas is collected and buried by the sublimate getter material; and

means for sequentially connecting another pellet to said source of potential when the pellet serving as anode is exhausted.

112. A getter-ion vacuum pump comprising:

a housing from which gas is to be removed;

a hollow cylindrical electrode disposed within said housing and defining an ion collector, the point of intersection of the axis of said cylindrical electrode with the plane of one of its end faces defining a reference point;

at least one powered electron emissive filament disposed within the interior of said cylindrical electrode, said electrode being maintained at a potential which is negative with respect to said filament to focus the emitted electrode along the axis of said cylindrical electrode to form an axially focussed electron stream;

at least two pellets of getter material disposed within a spherical space centered at said reference point and having a radius equal to the radius of said cylindrical 3 electrode and arranged symmetrically with respect to axis of said electron stream;

means for independently connecting each of said pellets to a source of potential which is positive with respect to said filament to serve as an anode, said stream of electrons sublimating the pellet serving as anode by electron bombardment and ionizing the gas molecules within said housing for driving said ionized gas to the wall of said housing and ion collector where the gas is collected and buried by the sublimate getter material; and

means for sequentially connecting another pellet to said source of potential when the pellet serving as anode is exhausted.

13. A getter-ion vacuum pump comprising:

an enclosure defining an interior space having interior walls and including a port for connecting said interior space to a device to be pumped;

a hollow elongated electrode disposed within said interior space and maintained at a reference potential for collecting ions and for focussing electrons;

an electron emitting filament substantially centrally disposed within said electrode and maintained at a potential which is positive with respect to said reference potential to serve as a cathode, said filament potential being selected for focussing of the emitted electrons by said electrode into an electron stream; and

at least one pellet of a getter material, for forming a sublimate for deposit upon said interior walls and the walls of said electrode, disposed within the focussed electron stream, said pellet being maintained at a potential which is positive with respect to said filament to serve as anode and to be sublimed by the emitted electrons by bombardment.

14. A getter-ion vacuum pump comprising:

an enclosure defining an interior space and including a port for connecting said interior space to a device to be pumped;

a hollow right cylindrical electrode disposed within said interior space and maintained at a reference potential to collect ions and to focus electrons, the point of intersection of the axis of said electrode with the plane of one end face defining a reference point; i

an electron emitting filament substantially centrally disposed within the interior of said cylindrical electrode and maintained at a potential which is positive with respect to said reference potential to serve as a cathode, the filament potential being selected for optimum focussing of the electrons emitted from said filament by said electrode; and

at least one pellet of a getter material for forming a sublimate for deposit upon the walls of said interior space and cylindrical electrode, said pellet being disposed within a spherical space centered at said reference point and having a radius equal to the radius of said cylindrical electrode, said pellet being connectible to a potential which is positive with respect to said filament to serve as sublimation target for the electrons emitted by said filament and focussed by said cylindrical electrode.

References Cited UNITED STATES PATENTS ROBERT M. WALKER, Primary Examiner. 

1. A GETTER-ION VACUUM PUMP COMPRISING: AN ENCLOSURE DEFINING AN INTERIOR SPACE HAVING INTERIOR WALLS AND INCLUDING A PORT FOR CONNECTING SAID INTERIOR SPACE TO A DEVICE TO BE PUMPED; A HOLLOW ELONGATED ELECTRODE DISPOSED WITHIN SAID INTERIOR SPACE AND MAINTAINED AT A REFERENCE POTENTIAL FOR COLLECTING IONS AND FOR FOCUSING ELECTRONS; AN ELECTON EMITTING FILAMENT SUBSTANTIALLY CENTRALLY DISPOSED WITHIN SAID ELECTRODE AND MAINTAINED AT A POTENTIAL WHICH IS POSITIVE WITH RESPECT TO SAID REFERENCE POTENTIAL TO SERVE AS A CATHODE, SAID FILAMENT POTENTIAL BEING SELECTED FOR FOCUSSING OF THE EMITTED ELECTRONS BY SAID ELECTRODE INTO AN ELECTRON STREAM; AT LEAST TWO PELLETS OF A GETTER MATERIAL, FOR FORMING A SUBLIMATE FOR DEPOSIT UPON SAID INTERIOR WALLS AND THE WALLS OF SAID ELECTRODE, DISPOSED WITHIN THE ELECTRON STREAM; AND MEANS FOR INDEPENDENTLY CONNECTING EACH OF SAID PELLETS, ONE AT A TIME, TO A POTENTIAL WHICH IS POSITIVE WITH RESPECT TO SAID FILAMENT TO SERVE AS AN ANODE AND TO BE SUBLIMED BY THE EMITTED ELECTRONS BY BOMBARDMENT. 